We will identify and characterize all the MHC Class II beta chain genes and pseudogenes in a cell line containing a single copy of the MHC. The sequence information obtained will be used to construct a series of vectors that permit rapid construction of hybrid human-murine class II genes containing the polymorphic first domain of each HLA class II gene of any individual. These hybrid genes will be used to transfect lymphocytes for murine immunizations and thus production of monoclonal antibodies directed against allele specific epitopes of the human genes. The methods will be applied to obtain a full characterization of the genes expressed in DR3 and DR4 individuals, to evaluate the heterogeneity within these serologic types, and to prepare monoclonal antisera subdividing the DR3 and DR4 groups.